ElectroMechanical Systems include Micro ElectroMechanical Systems (MEMS) structures and Nano ElectroMechanical Systems (NEMS). MEMS and NEMS structures are used in a wide variety of applications including, for example, MEMS accelerometers in cars for airbag deployment or in consumer electronic devices such as game controllers, MEMS gyroscopes used in cars to detect yaw, optical switching, bio-MEMS applications, MEMS loudspeakers, inkjet printers and RF MEMS components such as antenna phase shifters. Advantages of using MEMS structures include they have mechanical properties but have small dimensions and can be manufactured using existing semiconductor processing technologies.
A MEMS transducer device, which may be used as an actuator or sensor, may include a movable structure fabricated on a semiconductor substrate including at least one mechanical stack comprising one or more mechanical layers of a material such as silicon or silicon nitride and at least one functional/actuating stack whose function is to facilitate the movement of the mechanical stack on actuation of the device. The actuating stack comprises one or more layers whose arrangement and function in relation to the mechanical stack depends on the type of MEMS transducer device. For example, in an electrostatic actuated device, the actuating stack comprises a stationary electrode that cooperates with a movable electrode formed on a mechanical layer to facilitate movement of the mechanical layer and movable electrode. In a magnetic actuated device, the functional stack comprises a magnetic layer which is arranged to apply an external force to the movable mechanical stack in cooperation with an external magnet. The actuating stack can also be a multi-layered stack including at least one actuating layer of a material such as a piezoelectric or a magnetic material formed over a mechanical stack such as a mechanical beam or cantilever. Due to its electromechanical conversion properties, lead zirconate titanate (Pb[ZrxTi1-x]O3 with 0<x<1) which is generally known as PZT, is the most commonly used piezoelectric material in MEMS devices. In the case of a piezoelectrically actuated MEMS switch device such as that shown in FIG. 1, the multi-layer movable structure includes a PZT film 2 formed over a cantilever or beam 4 (which may be a silicon nitride or a silicon oxide cantilever) and electrodes 6 and 8 (which may be platinum electrodes) formed on either side of the PZT film 2 for applying a voltage across the PZT film. Contacts 10 and 12 provide the switch contacts of the device. As is well known, by applying appropriate voltages across the PZT film, the PZT film expands or contracts depending on the applied voltage by piezoelectricity which applies stress to the cantilever and results in the cantilever being deflected orthogonally (in a direction perpendicular to the stack) to open or close the MEMS switch device.
An article entitled ‘Design, fabrication and RF performances of two different types of piezoelectrically actuated Ohmic MEMS switches’ by Hee-Chul Lee, Jae-Hyoung Park, Jae-Yeong Park, Hyo-Jin Nam and Jong-Uk Bu in Journal of Micromechanics and Microengineering 15 (2005), pages 21098-2104, describes a piezoelectric actuated RF MEMS switch having a PZT capacitor formed on a cantilever.
US patent application no. 2005/0127792 describes a multi-layered piezoelectric switch for tunable electronic components comprising multiple piezoelectric layers, and metal layers alternated with the piezoelectric layers on a cantilever. Thus, this device uses stacked piezoelectric capacitors to form a piezoelectric actuated switch.
For MEMs transducer devices having a movable structure with at least one free end (for example, clamped with a single anchor) and being composed of multi-layer materials stacked together, the deflection of the movable structure can vary with temperature change due to the different values of the Coefficient of Thermal Expansion (CTE) for the different materials which form the movable structure as in a bimetallic strip. For example, for the piezoelectric actuated transducer of FIG. 1, the layers including the platinum (Pt) electrode 6, PZT film 2, and the platinum (Pt) electrode 8 of FIG. 1 will have a CTE of approximately 9.5 ppm/° C. compared to a CTE of 2-3 ppm/° C. of the silicon nitride cantilever 4. Thus, when the operation temperature changes, the Pt/PZT/Pt layers will expand (or contract) differently than the silicon nitride cantilever which results in changes in the transducers orthogonal deflection and thus, its performance. For example, for operation temperature changes over a 120° range, the piezoelectric MEMS switch device of FIG. 1 can experience a total deflection excursion of 7 μm. With large changes in the transducers deflection, the device may be made inoperable: for example, in the MEMS switch device of FIG. 1, the deflection due to temperature variations may cause the switch to be opened when it should be closed.
The same effect is seen in electrostatic switch devices having a movable structure with at least one free end and composed of a movable mechanical stack and a movable electrode layer formed on the moveable mechanical stack as part of the functional stack. The difference of CTE of the materials of the two layers can produce a thermal induced actuation.
The deflection described for a movable structure with at least one free end is due to the bending effect of a mechanical moment or force due to the multi-layer stack. The mechanical moment or force is typically referred to as the bending moment. This bending moment can have the same effect on other movable structures, such as, for example, clamped structures where the bending moment, due to a multilayer stack, is not present along the full structure. Such clamped structures include transducer devices having a movable structure (such as a mechanical layer or membrane) which is supported or clamped at ends of the movable structure and an actuating structure (such as a piezoelectric, electrostrictive or magnetostrictive actuating stack) located at the ends or at the centre of the movable structure. The actuating structure has a bending effect or induces a bending moment on the movable structure which causes the movable structure to move. As with the free end movable structures described above, the bending moment induced in such clamped structures may also vary with temperature variations.
It is known to provide thermal compensation in electrostatic switch devices by having additional layers which are identical and symmetrical to the movable electrode so as to compensate for the thermal behaviour of the movable structure.
For example, U.S. Pat. No. 6,746,891 describes a trilayered beam MEMS switch device which is actuated by an electrostatic charge. When a voltage is applied across a stationary electrode on a substrate and an opposing movable electrode on a movable beam, an equal and opposite charge is generated on the stationary electrode and movable electrode. The charge distribution on the opposing electrodes produces an electrostatic force that is balanced by the elastic forces on the now deformed beam. As the voltage is increased, the charge increases in a non-uniform and non-linear fashion across the surface of the beam until a stability point is reached. The stability point is defined by the inability of the elastic forces to maintain equilibrium with the electrostatic forces and the beam snaps through to establish contact between two switch contact pads. This patent describes how an electrode interconnect is formed on the beam, which electrode interconnect is a structural match or structurally similar to the movable electrode so as to provide robustness to film stress and temperature induced beam deformation. In one embodiment, this patent teaches that the electrode interconnect is fabricated of the same material and dimensioned the same in order to provide mechanical balance.
For the electrostatic actuated device, only one (movable) electrode layer, combined with a stationary electrode layer, is required for the device to function. Therefore, it is not too complex to use a symmetrical tri-layered structure to realize the thermal/stress balance. For a more complicated device having multiple layers, such as piezoelectric actuated device, at least three layers (electrode/PZT/electrode) form the functional/actuating stack and a mechanical beam layer forms the mechanical stack. This makes the thermal balance more difficult to be met. Theoretically, the same symmetrical approach as used in the electrostatic actuated device can be used in an attempt to achieve thermal balance: that is, the same three layers can be deposited on the opposite side of the mechanical beam layer. In reality, however, this is complicated by manufacturing process variations. More layers mean more processing steps and larger variations, resulting in higher cost and less reproducibility. Also, the presence of the PZT layer before mechanical beam deposition may not be allowed due to serious contamination concerns.
In view of process constraints, it is not always possible to put the same material on both sides of the mechanical stack, due to serious contamination problems or because of process conditions. For example, a metallic layer used as compensation for a metal electrode on top of a mechanical layer and made before the mechanical layer may not be compatible with the temperature deposition of the material of the mechanical layer. Thus, even for an electrostatic actuated device, there is a need to propose a solution to have improved thermal stability without using a symmetrical movable structure.